1. Field of the Invention
The present invention relates to a posthetic spinal annulus to be used in combination with a disc nucleus and more specifically, it relates to a surgically implantable intradiscal prosthesis and the method of manufacture thereof.
2. Description of Related Art
The vertebral spine is a complex arrangement of many structures, with many facets (areas of specially cushioned apposition). The vertebral bones are twenty-four in number, not including the sacrum, and they gradually vary in size and shape and load distribution from the cervical to the thoracic to the lower lumbar vertebrae. The vertebrae, amazingly, are very different between the first cervical and the last lumbar vertebra. Nonetheless, as shown in FIG. 1, the bony vertebral bodies 10 of the spine are each separated by a relatively soft intervertebral disc 12, which acts as a joint, allowing flexion, extension, lateral bending, and axial rotation. Fibrous tissues, emulating scar tissues, may act somewhat similarly to the bonding elements that make up the ligaments of the spine, as well as the outer portions of the relatively soft intervertebral discs. If a synthetic vertebral disc were to be placed to repair one that is naturally damaged, it would be beneficial to have the participation of these fibrous fixing elements in a relatively controlled and maximized fashion.
The typical vertebra has a thick interiorly located bone mass called the vertebral body (with a neural vertebral arch that arises near the posterior surface of the vertebral body). The intervertebral disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motion within the vertebral segments of the axial skeleton. The normal disc is a unique mixed structure, comprised of three component tissues, including the nucleus pulposis (nucleus), the annulus fibrosus (annulus), and the two opposing vertebral end plates. The two vertebral end plates are each composed of thin cartilage overlying a thin layer of hard cortical bone, which attaches to a spongy, richly vascular cancellous bone of the vertebral body. The vertebral end plates thus serve to attach the adjacent vertebra to the disc. In other words, a transition zone is created by the end plates between the malleable disc and the bony vertebra.
The annulus of the disc is a tough outer fibrous ring that binds together the adjacent vertebrae. The fibrous portion is much like a laminated automobile tire measuring about 10 to 15 mm in height and about 15 to 20 mm in thickness. Fibers of the annulus consist of 15 to 20 overlapping multiple plies and are attached at the superior and inferior vertebral body at a roughly 30-degree angle in both directions. This configuration particularly resists torsion as about half of the angulated fibers will tighten when the vertebrae rotate in either direction relative to each other.
Inside the annulus there is a relatively liquid core, the nucleus. The healthy natural nucleus has a high water content and aids in the load bearing and cushioning properties of the spinal disc; however, the spinal disc may be displaced or damaged due to trauma or disease. A disc herniation occurs when the annulus fibers are weakened or torn and the nucleus becomes permanently stressed, extended, or extruded out of its normal internal annular confines. A herniated or slipped nucleus can compress a spinal nerve posteriorly, resulting in pain, loss of muscle control, or even paralysis. Alternatively, in disc degeneration the nucleus loses its water binding capacity and deflates as though the air had been let out of a tire. Subsequently, height of the nucleus decreases, causing the annulus to buckle in areas where the laminated plies are loosely bonded. As the overlapping laminated plies of the annulus begin to buckle and separate, either circumferential or radial annular tares may occur and contribute to persistent and disabling pain. Adjacent ancillary spinal facet joints to the rear may also be forced into an overriding position, which may cause additional back pain as tissues are damaged due to irregular contact and force application.
It is an object of the present invention to provide a prosthetic spinal disc made of biocompatible material and having a nucleus with a material density that is different from the material density of the disc annulus.
It is another object of the invention to provide a prosthetic spinal disc that includes materials that vary in density.
It is another object of the invention to provide a prosthetic spinal disc including a nucleus and an annulus that may be optionally shaped, using a computed tomography (CT) scan, ultrasound imaging and/or magnetic resonance imaging (MRI) where the nucleus provides a central open reception area for the introduction of a central supporting fluid or gel.
Still another object of the present invention is to provide a prosthetic spinal disc having an optional fiber optic carriage that is used to transmit wavelengths of light which will stimulate tissue ingrowth from the supported vertebrae.
Another object of the invention is to provide a prosthetic spinal disc including an annulus and a nucleus having internal baffling to provide additional stabilization.
The invention-is a prosthetic spinal disc including a nucleus and an annulus that may be optionally shaped, using a computed tomography (CT) scan, ultrasound imaging and/or magnetic resonance imaging (MRI). The nucleus provides a central open reception area for the introduction of a central supporting fluid or gel. An optional fiber optic carriage or alternatively hollow continuous porosities is used to provide tissue ingrowth from the supported vertebrae and optional internal baffling provides additional stabilization.
The use of computed tomography (CT), ultrasound imaging and/or magnetic resonance imaging (MRI) enables the use of the bony irregularities of the vertebrae to create a tight fit for the spinal disc. The present invention provides stereotactic forming, either manually or automated, using a CAT scanner, MRI or three-dimensional ultrasound in order to shape a prosthetic spinal disc. This will simulate the outer portion of the natural disc (annulus). An overlapping contoured lip protrudes from the edge of the prosthetic annulus to cap and contain the movement of the vertebrae above and below the prosthetic spinal disc.
An alternative aspect of the invention utilizes certain wavelengths of laser light to cause ingrowth of beneficial tissues and cells. Electromagnetic energy may be transported through fiber optics to portions of the vertebral disc for extended periods of time, using the disc as an actual carrying mechanism. Ingrowth of fibrocartilagenous tissues into porosities may be generated to lock the disc in place.
The prosthetic nucleus and annulus of the present invention may be formed as an integral unit, where the fiber optics pass through the nucleus and then into the annulus. Alternately, the fiber optics may only pass through the annulus portion. Internal baffles may divide portions of the annulus, which provides a central open nuclear reception area for the introduction of a central supporting fluid or gel. Baffles or septae serve to restrain the movement and reduce shock to portions of the prosthetic annulus.
A further variant of this disc contains a silicone or relatively gelatinous polymer in the nucleus that simulates the nucleus of the natural disc. Additionally, the implant may contain a rigid lip around the edge in order to help hold it in place between the superior and inferior vertebral bodies. It may be desirable that the external few millimeters of the vertebra and disc be porous. The lip may also include porosities. The porosities may be relatively oriented in a circular pattern in order to mimic the natural positioning of the elements that hold on to the disc from the bony elements. Additionally, growth factors or living cells may be incorporated into the polymer surrounding the holes in order to aid cell growth.